Add docs for stable-compute-uuid behaviors

This adds some admin guide documentation about the stable compute_id
file. It covers upgrade, greenfield generation, and greenfield
pre-provisioning by deployment tools.

Related to blueprint stable-compute-uuid

Change-Id: I078b3f9e1919f2008628dc7b889e8696f1f6159a

Author: kk7ds

doc/source/admin/compute-node-identification.rst

@@ -0,0 +1,83 @@
+===========================
+Compute Node Identification
+===========================
+
+Nova requires that compute nodes maintain a constant and consistent identity
+during their lifecycle. With the exception of the ironic driver, starting in
+the 2023.1 release, this is achieved by use of a file containing the node
+unique identifier that is persisted on disk. Prior to 2023.1, a combination of
+the compute node's hostname and the :oslo.config:option:`host` value in the
+configuration file were used.
+
+The 2023.1 and later compute node identification file must remain unchanged
+during the lifecycle of the compute node. Changing the value or removing the
+file will result in a failure to start and may require advanced techniques
+for recovery. The file is read once at `nova-compute`` startup, at which point
+it is validated for formatting and the corresponding node is located or
+created in the database.
+
+.. note::
+
+    Even after 2023.1, the compute node's hostname may not be changed after
+    the initial registration with the controller nodes, it is just not used
+    as the primary method for identification.
+
+The behavior of ``nova-compute`` is different when using the ironic driver,
+as the (UUID-based) identity and mapping of compute nodes to compute manager
+service hosts is dynamic. In that case, no single node identity is maintained
+by the compute host and thus no identity file is read or written. Thus none
+of the sections below apply to hosts with :oslo.config:option:`compute_driver`
+set to `ironic`.
+
+Self-provisioning of the node identity
+--------------------------------------
+
+By default, ``nova-compute`` will automatically generate and write a UUID to
+disk the first time it starts up, and will use that going forward as its
+stable identity. Using the :oslo.config:option:`state_path`
+(which is ``/var/lib/nova`` on most systems), a ``compute_id`` file will be
+created with a generated UUID.
+
+Since this file (and it's parent directory) is writable by nova, it may be
+desirable to move this to one of the other locations that nova looks for the
+identification file.
+
+Deployment provisioning of the node identity
+--------------------------------------------
+
+In addition to the location mentioned above, nova will also search the parent
+directories of any config file in use (either the defaults or provided on
+the command line) for a ``compute_id`` file. Thus, a deployment tool may, on
+most systems, pre-provision the node's UUID by writing one to
+``/etc/nova/compute_id``.
+
+The contents of the file should be a single UUID in canonical textual
+representation with no additional whitespace or other characters. The following
+should work on most Linux systems:
+
+.. code-block:: shell
+
+    $ uuidgen > /etc/nova/compute_id
+
+.. note::
+
+    **Do not** execute the above command blindly in every run of a deployment
+    tool, as that will result in overwriting the ``compute_id`` file each time,
+    which *will* prevent nova from working properly.
+
+Upgrading from pre-2023.1
+-------------------------
+
+Before release 2023.1, ``nova-compute`` only used the hostname (combined with
+:oslo.config:option:`host`, if set) to identify its compute node objects in
+the database. When upgrading from a prior release, the compute node will
+perform a one-time migration of the hostname-matched compute node UUID to the
+``compute_id`` file in the :oslo.config:option:`state_path` location.
+
+.. note::
+
+    It is imperative that you allow the above migration to run and complete on
+    compute nodes that are being upgraded. Skipping this step by
+    pre-provisioning a ``compute_id`` file before the upgrade will **not** work
+    and will be equivalent to changing the compute node UUID after it has
+    already been created once.

doc/source/admin/index.rst

@@ -206,6 +206,7 @@ instance for these kind of workloads.
    secure-boot
    sev
    managing-resource-providers
+   compute-node-identification
    resource-limits
    cpu-models
    libvirt-misc

doc/source/cli/nova-compute.rst

@@ -41,6 +41,8 @@ Files
 * ``/etc/nova/policy.d/``
 * ``/etc/nova/rootwrap.conf``
 * ``/etc/nova/rootwrap.d/``
+* ``/etc/nova/compute_id``
+* ``/var/lib/nova/compute_id``
 
 See Also
 ========